Process for the preparation of globular sodium bisulfate

ABSTRACT

PRECIPITATION OF IRON SALTS IN MOLTEN SODIUM BISUFATE IS PREVENTED BY THE STEPS OF ADDING WATER TO A MOLTEN SODIUM BISULFATE PROCESS STREAM, THEREBY COOLING THIS MOLTEN SODIUM BISULFATE PROCESS STREAM TO A TEMPERATURE BELOW 500*F. BUT MAINTAINING IT ABOVE THE CRYSTALLIZATION TEMPERATURE OF THE SODIUM BISULFATE, CONTACTING THE COOLED SODIUM BISULFATE-WATER MIXTURES WITH METALLIC ZINC, AND THEN SPRAY-FORMING THE MIXTURE INTO GLOBULES.

United States Patent 3,690,825 PROCESS FOR THE PREPARATION OF GLOBULAR SODIUM BISULFATE Reuben C. Ott, Bay Village, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Continuation-impart of application Ser. No. 671,603, Sept. 29, 1967. This application Oct. 30, 1970,

Ser. No. 85,740

Int. Cl. C01d 5/02 US. Cl. 423-264 5 Claims ABSTRACT OF THE DISCLOSURE Precipitation of iron salts in molten sodium bisulfate is prevented by the steps of adding water to a molten sodium bisulfate process stream, thereby cooling this molten sodium bisulfate process stream to a temperature below 500 F. but maintaining it above the crystallization temperature of the sodium bisulfate, contacting the cooled sodium bisulfate-water mixtures with metallic zinc, and then spray-forming the mixture into globules.

CROSS-REFERENCES TO RELATED APPLICATION This application is a continuation-in-part of an application of Reuben C. Ott, Ser. No. 671,603, filed on Sept. 29, 1967, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to a process for the prevention of precipitation of iron salts in a molten sodium bisulfate process stream.

The presence of iron salts as an impurity in sodium bisulfate has long been a problem in the commercial production of sodium bisulfate. The presence of this impurity results in an undesirable discoloration of the product. Since sodium bisulfate is used as an ingredient in household cleaners and disinfectants it is desirable to produce a product with a pleasing appearance. It was known that the presence of the iron impurities in the final product could be masked by adding a reducing agent to the molten bisulfate process stream. This resulted in the decoloration of the iron salts giving a product with an acceptable appearance.

It was also known that when a reducing agent was added to decolorize the iron present in the product stream, a new problem developed. The iron salts became insoluble in the molten sodium bisulfate process stream. The presence of this precipitate in the molten sodium bisulfate process stream resulted in frequent clogging of the spray apparatus used to form the sodium bisulfate into globules. Therefore, when the sodium bisulfate was decolorized, it was considered part of the product overhead that the process apparatus would have to be shut down for cleaning at frequent intervals to prevent or correct problems due to clogging caused by the crystallized iron salts.

This invention has eliminated both the clogging of the process apparatus by insoluble iron salts present as an impurity in sodium bisulfate and at the same time improving the appearance of the final product by causing the iron salts to change color from brown to an innocuous off-white.

Patented Sept. 12, 1972 ice SUMMARY OF THE INVENTION DETAILED DESCRIPTION OF THE INVENTION The sodium bisulfate is produced by the reaction of sodium chloride and sulfuric acid at a temperature of from about 575 to 620 F. resulting in the production of anhydrous HCl and molten sodium bisulfate.

Due to the fact that caking or cementation of ground sodium bisulfate is a problem, the molten sodium bisulfate is formed into small globules which do not cake upon standing. These globules can be formed by spraying the molten sodium bisulfate in an air stream which hardens it into globular particles.

The molten sodium bisulfate produced as described above often contains iron salts as an impurity in the molten sodium bisulfate process stream. These iron salts cause an undesirable discoloration of the product if no steps are taken to effect a color change.

When the process of my invention is employed, the producer of sodium bisulfate is no longer forced to choose between producing a pleasing product with high production costs or lower production costs with the result that an unappealing product is produced.

To the molten sodium bisulfate process stream is added about 5 to 10 percent by weight of water. This water is added to react with any sodium pyrosulfate which may have formed, and to reduce the temperature of the molten sodium bisulfate, through flash evaporation, to a temperature of from about 360 to 450 F. This water addition also results in a sodium bisulfate which has desirable crystal characteristics. For optimum results, it is preferred that the molten system be adjusted to a temperature of from 400 to 420 F. In a continuous process, the amount of water required is about 5.7-6% by weight. Generally, the amount of Water needed is based on the amount necessary for decomposition of sodium pyrosulfate formed in the reaction to sodium bisulfate, plus the amount lost by evaporation, plus an excess of about 1.7-1.9% based on the weight of the melt to control the solidification temperature of the melt, which is maintained within the range of 330-340 F.

When the molten system has been adjusted to a temperature within the above range, the molten system is contacted with about 0.02 to 0.05 percent by weight of metallic zinc. When only small amounts of iron impurities are present in the molten sodium bisulfate, the amount of zinc can be reduced below the above lower limit, to about 0.01 weight percent. Exceeding the upper limit normally would be wasteful and unnecessary. Nevertheless, zinc concentrations as high as 0.5% can be used. Concentrations higher than 0.5% would be undesirable and would be considered impurities in the final product. While zinc dust can be used, it is more practical to employ larger pieces of zinc, such as lumps, slabs, or large granules, such as zinc shot. Zinc dust under the conditions of the reaction reacts so fast that dangerous concentrations of hydrogen may be reached, with attending explosion hazards.

When this sequence of steps is followed, the iron salts remain dissolved in the molten sodium bisulfate and are also decolorized. This means that the molten sodium bisulfate can be formed into globules by spraying without having the apparatus frequently clogged due to the presence of the insoluble iron salts, while at the same time a product is produced that is free from discoloration.

The novel aspects of this process are hereinafter more fully elaborated in an effort to convey a clearer understanding of my invention to the reader. As explained above, the application of a reducing agent, such as zinc, to correct the discoloration of the sodium bisulfate product has been heretofore employed in the art. Prior to my discovery, however, it was not known that the addition of water to the molten sodium bisulfate process stream prior to the reduction step would eliminate the precipitation of the iron impurities.

I have further discovered that the temperature of the sodium bisulfate must be in the range of from about 360 to 450 F. when the reducing agent is added to assure that no precipitate will form. Indeed, those engaged in the manufacture of this product have looked upon the presence of this iron precipitate in the molten sodium bisulfate system as an inherent problem encountered when a reducing agent is employed and a part of the normal characteristics of this method of manufacture.

Essentially, my invention is a process improvement in the production of globular sodium bisulfate. In this process sodium bisulfate is formed as a result of the reaction of sodium chloride and sulfuric acid. This reaction is run at a temperature sufiicient to result in the formation of molten sodium bisulfate. It is most convenient to operate at a temperature of from about 575 to 620 F.

My improvement consists of first treating the molten sodium bisulfate process stream with water. The treated sodium bisulfate, while at a temperature of from about 350 to 450 F., is then contacted with metallic zinc to decolorize the iron impurities which are present. The treated reduced molten sodium bisulfate is then sprayed in an air stream which hardens it into white, globular particles.

The mechanism producing this result is not fully understood, but it is known that deviation from the order hereinabove specified does not result in the substantially complete elimination of precipitated iron salts in the molten system.

The following examples are presented to illustrate the process of my invention to better enable those skilled in the art to understand my invention.

EXAMPLE 1 Molten sodium bisulfate is prepared in a suitable reactor by reacting sodium chloride with sulfuric acid at 600 F. One thousand parts of this molten material is fed from the reactor into a treating tank to which is added 58.15 parts by weight of water and this mixture of molten sodium bisulfate and water is adjusted to a temperature of approximately 405 F. Then 0.3 part by weight of zinc dust is added to this mixture.

The treating tank is partitioned so that the water, which is added first, is incorporated into the molten sodium bisulfate before the zinc addition point.

After the addition of the zinc dust, the yellow iron impurities present are decolori'zed and no precipitate is observed to form. This mixture is then fed into a spray dispersion unit, where it is spray-formed and cooled as white globules with a pale green cast.

EXAMPLE 2 Sodium chloride and sulfuric acid are continuously added in approximate molar proportions to a reactor maintained at 600-620 F. Hydrogen chloride gas and molten sodium bisulfate are obtained as the reaction products. The respective amounts of the ingredients are adjusted so that the acidity of the melt is maintained in the range corresponding to about 37 to 39 weight percent of sulfuric acid. The molten sodium bisulate is conveyed to 4 the inner section of a cylindrical, vertical tank which is divided by a concentric, cylindrical dam into two sections in such a way that the melt is forced to underflow from the center section to the outer, annular section.

The first section is equipped with a surface agitator, and the size of the tank is so chosen that the total residence time in both sections together is about 3 to 6 minutes. Water and molten sodium bisulfate are added simultaneously to the first section, the amount of Water being about 0.7 gallon per lbs. of melt. The temperature of the melt is lowered thereby from about 600 F. to about 440 F., and the solidification temperature is brought to the range of 330 to 340 F. The material displaced from the first section undeflows the dam to the second section in which zinc metal in lump or slab form is placed. In the plant, commercial zinc slab weighing about 65 pounds or commercial molded zinc lumps of the size approximately 4 inches by 2 /2 inches by inch are used. The rate of solution of the zinc is about 0.04 lb. per 100 lbs. of melt. Periodically, additional pieces of zinc are added to maintain an excess of zinc in the tank. The treated melt is then processed in equipment which converts it to the white, globular finished product.

In the above reactor, the rate of flow of the melt was varied from about 5,000 to about 20,000 lbs. per hour, and the same results were obtained. It will be readily apparent, however, that the design of the equipment is not critical, an important requirement being that good contact between the melt and the metallic zinc be maintained.

I have found in laboratory equipment in a batch process that when the order of addition to the treating tank was reversed so that the melt from the reactor first contacted the zinc, an insoluble precipitate was formed. This precipitate was formed even when the melt was first cooled to about 400-440 F. by some other means than by evaporative cooling with water. While the direction of flow could not be reversed in the plant equipment, the same result was observed when the zinc was added to the stream of molten sodium bisulfate before the addition of Water, i.e., an insoluble precipitate was formed.

Since the addition of water to the melt must be done under good agitation conditions, the agitation of the first section of the tank is very important and requires at least half of the total residence time of the melt in the treating tank. The total reaction time in contact with the zinc thus is about 1.5-3 minutes. Although under the plant conditions the melt still goes through a pumping system and storage tank prior to the final solidification, I have found in laboratory equipment that both the reaction of Zinc lumps with sodium bisulfate and the resulting whitening effect are immediate. In the laboratory equipment, the melt was cooled as soon as zinc had been added to it, and the maximum contact time prior to complete solidification was only 5-10 minutes.

The above experiments show clearly that my process eliminates the formation of insoluble contaminants; and that it is readily adaptable to a continuous plant process which has been satisfactorily carried out and practiced. The order of addition of water is critical, as can be seen from the experiments in which either the flow direction or the order of addition was reversed.

I claim:

1. In a process for preparing substantially white, globular sodium bisulfate from molten sodium bisulfate, said sodium bisulfate being formed by the reaction of sodium chloride and sulfuric acid at elevated temperatures, the improvement comprising the following sequential steps:

(a) adding to the molten sodium bisulfate an amount of water sufficient to decompose to sodium bisulfate any sodium pyrosulfate formed in the reaction, to reduce by evaporation the temperature of said sodium bisulfate to the range of about 360450 F., and to maintain in said sodium bisulfate an excess of about 1.7-1.9 percent of water based on the weight of said bisulfate; and allowing part of the water to flash-evaporate, thereby lowering the temperature of said bisulfate to about 360-450" F.;

(b) contacting said molten sodium bisulfate with metallic zinc in the form of slab, lumps, or large granules; and

(c) forming the treated molten sodium bisulfate into globules and cooling.

2. The process of claim 1 wherein the molten sodium bisulfate is allowed to cool to a temperature of 400-420" F. prior to being contacted with metallic zinc.

3. The process of claim 1 that is carried out batchwise.

4. The process of claim 1 that is carried out in a continuous manner.

5. The process of claim 4 wherein the amount of water added is about 5.76 percent based on the weight of the molten sodium bisulfate.

References Cited UNITED STATES PATENTS 2,893,836 7/1959 Davis et a1. 23121 2,861,868 11/1958 Stites, Jr. et al. 23-121 1,671,866 5/1928 Linville et a1. 23-l21 OTHER REFERENCES I. W. Mellors Comprehensive Treatise on Inorganic 10 and Theoretical Chemistry, 1930 ed., page 445. Longmans, Green & Co., New York.

EDWARD STERN, Primary Examiner US. Cl. X.R. 23-1 D; 423-520 

